With the recent development of bicycles designed to be ridden off-road ("mountain bikes"), a jamming problem has emerged in the bicycle chain drive. The problem, sometimes referred to as chain-suck or chain-wrap, henceforth referred to herein as chain-wrap, involves the interaction of the drive chain and the front chainrings (the front toothed sprockets attached to the crank pedal arms and rotatably mounted in the bottom bracket).
For various reasons, discussed below, the chain will fail to release from the chainring at the appropriate point. The chain is carried until it is either pulled off by the spring tension of the derailleur or jammed between the chainring and the chainstay or chainstay mounted brake.
The problem of chain-wrap is not unique to mountain bikes, but the conditions in which the bikes are ridden and their design accentuates the problem over that of bikes designed for conventional road use.
Unlike most road bikes which have two, mountain bikes are generally fitted with three coaxial chainrings or sprocket wheels. The smallest being in the range of 24-28 teeth, the middle 34-38 teeth, and the largest 44-52 teeth.
There are two different modes of chain-wrap. The most common mode occurs when shifting between two chainrings, generally from the middle to the smallest chainring of vice versa. The chain is shifted from one chainring to the other by means of the front derailleur. For a period of time, the chain is partially on both chainrings. As the chain has limited lateral flexibility, this causes the chain to press up tightly to the chainrings which may cause the chain to stick or bind.
Chain wrap is also known to occur without shifting usually while in the middle or small chainring. Often rough terrain is involved.
There are many variables which alone or working with others seem to increase the occurrence of chain-wrap. A partial list includes the following:
(1) Dirty or unlubricated chain. As these bikes are often ridden in extremely muddy or dusty conditions, this is frequently the case.
(2) Poor chain line. Chain line refers to the positioning of the chainrings with respect to the gears in back. A poor chain line increases the lateral stress on the chain, increasing the likelihood of chain-wrap.
(3) Riding over bumpy terrain. Bumpy terrain tends to cause chain to move considerably. The lower section may momentarily bounce higher facilitating chain-wrap.
(4) Differences in equipment. Smaller chainrings result in more chain slack which must be taken up by the rear derailleur resulting in lower chain tension. There are also noticeable differences in frequency of chain-wrap between different brands of chains and chainrings, but all brands exhibit chain-wrap in use.
(5) Worn equipment. As the chainrings wear, grooves are worn into the leading edge of the individual teeth resulting in hooked teeth which may prevent proper chain release. Wear is relatively rapid due to the dirty conditions of use.
(6) Shifting under unusually high pressures. Typical mountain bike terrain varies rapidly. Consequently, a shift is often initiated "late" (under heavy pedal pressure). The added pressure on the chain makes it less likely to release.
The damage caused by chain-wrap is variable. Generally the chain is jammed and must be forced out by back pedalling, or in extreme cases, removed with a tool. The chain, chainrings, frame and rear derailleur may be damaged. If chain-wrap occurs frequently, significant wear can occur on the chainstay where the chain wedges between it and the chainring.
Several steps can be and have been used to reduce the occurrence of chain-wrap. Simple measures such as proper bike adjustment, lubrication and proper riding style can significantly reduce the severity of chain-wrap.
The bicycle industry has responded with a few solutions. Shimano Corporation has a device called a Shark-tooth.RTM.. This is a piece of plastic attached to a brake that is mounted under the chainstays in an attempt to eliminate chain-wrap. The device guides the chain stuck to the chainrings so as to hit the brake squarely and not to slip off to the side. On some occasions this is enough to dislodge the chain but usually the chain stays on the chainrings, travels under the brake and still wedges into the space between the chainrings and chainstay. The industry, as a whole, is phasing out under-the-chainstay brakes. Another device mounted under the rear chainstay-mounted brake resembles a miniature guard rail protecting the brake from chain hits, but without apparent success at eliminating the chain wrap. A custom builder provided a flat plate mounted to the underside of his rear brake calipers, which the consumer was instructed to hacksaw and file into shape to fit the chainrings. When mounted on a typical bike, however, the plate only came close to the largest chainring, providing no benefit for the common occurrence of chain wrap in the middle and small chainrings. In fact, the plate is positioned so low as to interfere with the normal operation of the chain on the small chainring. The mounting holes are drilled slightly larger than the fastening bolts, allowing a small amount of adjustment of the resulting plate, but not enough to fit a variety of cranksets. This type of flat plate design appears to have chain interference and fit problems with normal bicycles.
Another response or solution has been to raise the chainstays to a positions above the chainrings. In this case the chain will not hit the raised chainstay but will wrap all the way around to become caught under the chain feeding onto the chainring through the front derailleur (on top). If the rider does not catch the problem immediately and continues pedalling, the rear derailleur runs out of tension adjustment and is damaged or pulled out of the frame.
Other disadvantages of raising the chainstays are structural. The bicycle receives its strength and rigidity from the "double triangle" formed in a typical frame. The tension in the driving chain is counteracted by compression in predominately the right chainstay, see my U.S. Pat. No. 4,621,827. As the two loading pounds are the bottom bracket assembly and the rear dropout, the chainstay, which goes almost straight between them, is most effective in resisting the forces and keeping deflection to a minimum. By raising the chainstays this advantage is lost requiring added reinforcement tubes in the frame and heavier tubing. Consequently, frames using raised chainstays are less rigid and heavier than a standard frame.
The object of this invention is to provide an anti-chain-wrap device to solve the afore mentioned problem, and more particularly, to provide an anti-chain wrap device which lifts the chain from the chainring teeth in the event that it has failed to release properly on its own.
The invention is applicable to a bicycle with the following characteristics:
(1) A chain drive system including rear sprockets to drive the rear wheel.
(2) A rear derailleur which maintains chain tension by means of a spring tensioned arm.
Bikes of this type experience an occasional failure of the chain to release the front chainring at the proper point (chain-wrap). This can lead to chain jamming and possible damage to components and the bike frame. The problem is magnified by smaller chainrings and dirtier conditions.
The anti-chain wrap device of the invention which is attached to the frame may also be called a chain-wrap prevention device. In its preferred embodiment it is bolted to the underside of the chainstay and has multiple "lifters", three "lifters" in the case of a three chainring front drive assembly, one for each chainring. The lifters are situated so that they are in close proximity to the chainrings. Some popular chainrings on the market are not round, but are elliptical or even more complex (Biopace). Trying to cause the chain to release by positioning a device such as a flat plate on the outside edge of the chainring occasionally allows the chain to jam even when the end of the tooth is in close proximity to the device using round chainrings, and when using the not round chainrings, during part of the revolution of the crank there will be a large gap between a device and the teeth, allowing the chain to easily jam. At the point at which the chain hits the plate, the chain was nearly at a right angle to the plate. Instead of freeing the chain from the chainring, the plate occasionally jammed the chain into the chainring solidly resulting in damage to both.
Preferably, the lifters are positioned to the side and raise the teeth for the two smaller rings so that they intersect the chainrings where the chain angle is low relative to the lifters. To dislodge a chain at a lower angle requires much less force. The lifters at this height easily lift the chain out of the chainring teeth. In the preferred embodiment, the lifters are larger than would be necessary for any one set of chainrings so as to accommodate a wide range of chainring sizes.
In its preferred embodiment, the anti-chain wrap device is a stamping out of 0.01 inch to 0.125 inch aluminum plate. The device has a flat area with three slotted bolt holes. The slotted holes adjustment of the device to fit different chainring and frame configurations and allows limited front to back movement, and substantial side-to-side and angular adjustment. Also optionally incorporated into the device is a cable guide for the rear derailleur and thus aids in installation of the device on a bicycle. The lifters for the middle and smallest chainring curve up and out of the flat part. Preferably, the lower half of the two raised lifters are connected for increased strength and rigidity. The curve in each lifter is sufficient to keep all but the top part clear of the chainrings. The lifter for the smallest chainring is as high as possible without hitting the chain in normal operation. The lifter for the middle chainring is lower to avoid the possibility of the chain jamming between the two lifters. The lifter for the large chainring is the shortest, but may be part of the flat mounting plate. The device is inexpensive and designed to be easily replaceable if the need should arise.